1. Field of the invention:
The present invention relates to recording media including audio tapes, video tapes, floppy disks, etc., in particular, to a magnetic powder for use as a material of such recording media, and to a method of producing the same.
2. Description of the Prior Art:
Recently, magnetic recording has been directed to high-density recording and digital recording. Conventionally, a so-called longitudinal magnetic recording system has been a major magnetic recording system, in which the easily magnetizable axis is in the plane of the magnetic recording medium. In accordance with this system, however, it is difficult to elevate the recording density of the recording medium, since the magnetized particles are so arranged that the magnetized directions thereof are repellent to one another with the increase of the recording density. Accordingly, a so-called perpendicular recording system has recently been considered for solving the problem, in which the recording is effected perpendicularly to the plane of the magnetic recording medium. (For example, refer to "High-density Magnetic Recording by Perpendicular Magnetization", Shunichi Iwasaki, Nikkei Electronics, Aug. 7, 1978, pages 100 to 111, Japan.)
For a recording medium suitable for use in the perpendicular magnetic recording system, it is essential to have an easily magnetizable axis in the perpendicular direction to the plane of the magnetic recording medium and the medium includes, for example, a thin film of an evaporation-plated alloy such as cobalt-chromium, and a hexagonal ferrite thin film. In particular, the hexagonal ferrite is expected to be useful as a coating-type perpendicular magnetic recording media (For example, refer to Japanese Laid-open Patent Application No. 60-193127.
As the hexagonal ferrite magnetic powder, there are known, for example, barrium ferrite, strontium ferrite and lead ferrite magnetic powders. Since these powders are hexagonal plate-like particles each of which has an easily magnetizable axis in the vertical direction to the hexagonal plane and the plane can easily become parallel to the magnetic recording medium plane when merely coated on the base of the medium, the magnetic powders are suitable for use in perpendicular magnetic recording media. For producing the hexagonal ferrite magnetic powder, certain methods have been established such as a hydrothermal reaction method and a glass crystallization method. (For example, refer to "Conditions for the Formation of Compounds Consisting of BaO and Fe.sub.2 O.sub.3 from Aqueous Suspensions", M. Kiyama, Bulletin of the Chemical Society of Japan, Vol. 49, No. 7 (July, 1976), pp. 1855-1860, or "Properties of Ba Ferrite Particles for Perpendicular Magnetic Recording Media", O. Kubo et al., IEEE Transactions on Magnetics, Vol. MAG-18, No. 6 (November 1982), pp. 1122-1124.)
However, although the magnetic recording medium formed by barium ferrite plate-like magnetic powder, for example, among the above-mentioned hexagonal ferrite, is excellent in magnetic recording characteristics in a short wavelength range, this has a defect in that this is inferior to a .gamma.-Fe.sub.2 O.sub.3 type magnetic powder-containing medium, which is a conventional medium for longitudinal magnetic recording system, in a long wavelength range. In addition, the barium ferrite plate-like magnetic powder has a large positive temperature coefficient of the coercive force Hc. Therefore, even when a signal is recorded on the barium ferrite-containing magnetic recording medium at normal temperature, the signal would not be reproduced optimally at a higher temperature because the coercive force would vary in accordance with the temperature elevation by environmental variation.
Moreover, although the above-mentioned hexagonal ferrite such as barium ferrite could sufficiently be used in the perpendicular magnetic recording medium due to the hexagonal plate-like particles each having an easily magnetizable axis in the perpendicular direction to the plane, the hexagonal ferrite which has conventionally been used has a small saturation magnetization and is poor in perpendicular orientation. Therefore, the conventional hexagonal ferrite cannot achieve sufficient recording and reproduction performance with the ferrite head which is generally used at present. In particular, the reproduction output level is largely reduced with increased recording density, and therefore, the recording density cannot sufficiently be elevated.